Study on potassium recovery of coal gasification process in supercritical water

Potassium carbonate (K₂CO₃) has been proved to be a good catalyst for coal gasification in supercritical water (SCW). However, catalyst deactivation occurs during supercritical water gasification (SCWG) of coal, resulting in large consumption and being unfriendly to the reaction system. In this paper, experiments on recovering potassium were investigated to find a suitable catalyst recovery method for coal gasification process in supercritical water. In the water washing method, prolonging the washing time, increasing the number of washes, increasing the water-slag ratio, and grinding the residue can effectively improve the recovery rate of potassium, which can reach 70.24 %. Acid washing can dissolve the oxides in the residue, and the maximum recovery rate of potassium can reach 89.75 %. The alkali washing method can replace the potassium in the solid residue with calcium through chemical reaction, and the total recovery rate of potassium can reach 96.51 %. It was found that the catalytic effect of the recovered liquid obtained by the water washing method and the alkali washing method in a carbon dioxide atmosphere is best. The potassium solution recovered by the acid washing method is the least effective. Because the liquids obtained by water washing and alkali washing in a carbon dioxide atmosphere can be regarded as potassium carbonate solutions, while the liquids obtained by the acid washing method is mainly potassium chloride solutions, so its catalytic effects is weaker. Through the catalytic effect of the recovered liquid, it can be seen the catalytic effect of the residue obtained: alkali washing method ≈ water washing method > acid washing method. A potassium recovery system coupled with a supercritical water gasification system is put forward through the in-depth study and comprehensive comparison of different methods.